Photovoltaic devices provide clean, quiet and reliable sources of electrical power. Because of shortages and environmental problems associated with fossil and nuclear fuels, as well as recent advances in technology which have significantly decreased the cost and increased the efficiencies of photovoltaic devices, solar generated electrical power is of growing commercial importance. The earliest photovoltaic devices were manufactured from single crystalline material. These devices were expensive, delicate, relatively bulky, and difficult to manufacture in large area configurations. Various techniques have now been developed for preparing thin film semiconductor materials which manifest electrical properties equivalent and in many instances superior, to their crystalline counterparts. These thin film materials may be readily deposited over very large areas and on a variety of substrates. Such alloys and techniques for their preparation are disclosed, for example, in U.S. Pat. Nos. 4,226,898 and 4,217,374. One important class of photovoltaic devices comprises a layer of intrinsic semiconductor material interposed between two oppositely doped semiconductor layers. Such devices are termed P-I-N or N-I-P devices depending on the order of the layers; the two terms shall be used interchangeably herein.
Glow discharge deposition comprises one particularly important class of techniques for the preparation of thin film semiconductor materials. In a glow discharge method, a process gas, typically at subatmospheric pressures, is energized by an electrical field so as to produce a plasma comprised of ionized and/or otherwise activated species derived from the process gas. The plasma acts to produce a semiconductor deposit on a substrate maintained in proximity thereto. Initially, such glow discharge deposition processes were energized by direct current, or, more commonly, by alternating current in the radio frequency range. While such techniques produce high quality, thin film semiconductor materials, deposition rates obtained thereby are quite low, and significant amounts of process gas are wasted. Attempts to raise the deposition rate, either by increasing the gas pressure or by greatly increasing the power density, result in the production of polymeric and oligomeric species which contaminate and degrade the semiconductor layers.
It has been found that microwave energy may be beneficially employed to energize a plasma in a glow discharge deposition process, and that a microwave energized plasma process is particularly advantageous for semiconductor fabrication. Very high rates of deposition may be achieved which are concomitant with a greatly enhanced process gas utilization. The application of microwave energy to glow discharge semiconductor deposition is disclosed, for example, in U.S. Pat. Nos. 4,517,223 and 4,619,729.
While microwave energized processes are attractive because of their high deposition rates and high rates of gas utilization, it has been found that the semiconductor materials deposited by these processes are generally of somewhat lower quality than those materials derived from an RF or DC energized plasma. Photovoltaic devices which include microwave deposited semiconductor layers have an overall efficiency which is generally lower than that of corresponding RF prepared devices. This is thought to be due to the fact that the higher energy microwaves create highly energized species, such as hydrogen ions. Energized hydrogen ions are necessarily present in any plasma derived from silane and other silicon hydrides (Si.sub.x H.sub.y) and these hydrogen ions can etch or otherwise interact with the surface of the deposited layer as it is being deposited. In the case of the more energetic hydrogen ions created by microwave-energized processes, it is speculated that the highly energetic hydrogen ions can actually do significant damage to the surface of the layer of deposited material. Additionally, the highly energized species produced by microwave deposition processes tend to deposit a semiconductor having a high density of states in the gap, a highly undesirable result.
It has been found that the interface between an intrinsic and a doped layer in semiconductor devices such as photovoltaic devices is particularly sensitive to deposition conditions. If a photovoltaic device of the P-I-N type is fabricated to have a doped-intrinsic layer interface wherein one of the layers is deposited in a microwave process and the other is deposited in a radio-frequency process, efficiency of the device will be degraded. Hence, it will be seen that a degraded interface between a doped semiconductor thin layer and an intrinsic semiconductor layer can result from microwave deposition of the intrinsic layer; and a degraded interface results in inferior performance of the entire device.
Clearly, it would be desirable to make thin film semiconductor devices employing a plasma deposition process which has the advantages of speed and efficiency characteristic of microwave deposition, and which also achieves devices which have the enhanced performance characteristics resulting from radio frequency deposition techniques. It would be particularly advantageous if such devices could be produced efficiently in a roll to roll process.